The present invention relates generally to three-phase alternating current supplies and more particularly to controls for correction of unbalanced voltages and/or currents supplied to three-phase electrical loads.
The single most costly factor in terms of wasted energy and loss of equipment life for three-phase motors is unbalanced phase voltages. Efficient use of materials and space in motor design has made the relationship between overheating and aging much more critical. The non-organic insulating materials which bring so many benefits to today's motors in terms of dielectric strength, higher operation temperatures, and more horsepower per pound, also bring with them some limitations. Cotton and varnish in the large frame motors of previous years would absorb thermal and mechanical abuse to a degree not permitted with glass fiber and silicon resins used in the motors of today. Less weight per horsepower is significant in relation to protection, in that thermal time constants are shorter, hence the heating rate from abnormal conditions have increased thus reducing thermal overload capability in the modern motor.
The life of a modern motor is not necessarily less than its predecessor if temperatures are kept within specified limits, but its rated temperature is already high and the same percent thermal overload in the modern motor as compared to the older motor results in a greater numerical temperature increase in the more modern motor.
The largest contributor to overheating in three-phase motors is unbalance of the voltages in a three-phase motor system. The temperature rise of a modern three-phase motor with an average voltage unbalance between phases of 10% means or dictates that a 200% rise in temperature will occur. This may be readily seen by reference to NEMA Standard MGI-14.33. When this factor is coupled with standard tests conducted on motor insulation, as documented in AIEE Specification 5101EEE 117, it reveals that a 10% increase in insulation temperature above design temperature causes motor insulation life to be halved.
Not only is the temperature rise caused by unbalanced phase voltages detrimental to motor life, it is also a basic cause of wasted energy given off in the form of heat, which is particularly critical in the present days of energy shortage.
Electric utilities, per industry standards, are only required to maintain voltages within plus or minus 10%. State and local standards may further limit the tolerance to plus or minus 5%, but there is no industry standard on voltage balance of a three-phase system other than the single-phase standards applied to each individual phase. Voltage unbalance of 5% to 10% or greater seems to be quite permissible, but fortunately, most utilities maintain a balance considerably better, but not to any controlled degree. A voltage unbalance of only 3.5% will cause a temperature rise of 25%.
Also, even though the voltage unbalance may be minimal at the point of origin or supply from the electric utility, voltage unbalance can greatly increase at remote locations of supply due to many different loads applied to the three-phase supply intermediate the point of original transmission and the ultimate remotely located load on the extreme ends of the transmission line. Also, current and power (voltage x current) deviations in different phases of a three-phase supply connected to a load also can cause energy loss and heat fatigue, shorting the life of the three-phase load and wasting expensive energy.